ACUTE PNEUMONIA

S. pneumoniae causes alpha hemolysis, but don’t confuse it with normal flora. A quick test to distinguish it from normal flora would be to look under a microscope. It always comes in two and so is sometimes called diplococci.

So how does the infection happen?

1. Well it starts as a droplet infection

2. Goes into the upper respiratory tract (S. pneumoniae live only in humans and their favorite place to live is the URT, they’re often found there with absolutely no harm to the host, for example after being a while in a military camp ~ 60% of soldiers test positive, but they aren’t sick.)

3. Can stay there for a while

4. Someone has a predisposing factor. (skull fracture , otitis, obstruction of eustacian tube , sinusitis. S. pneumoniae is the leading cause of ear infections in kids.) Flu or viral infection can also be a predisposing factor and brings the bacteria to the lungs {that’s why the flu is dangerous}

5. Invade tissue and initiate inflammation (inflammation is caused by the acidic capsule which the bacteria usually lose so the antibodies the body built up for the capsule are no good)

6. Lobar pneumonia: this has a very typical lesion that can be diagnosed by a radiologist looking at an x-ray

7. Most cases cured even without antibiotics, but

   1. Can get to pleural cavity

• Cause emphysema and even death if not treated.

2. If a person has a splenectomy, sickle cell anemia, or a defect in phagocyte formation the bacteria can get to the bloodstream and lead to endocarditis, meningitis, or arthritis.

Now onto assorted facts about the bacteria itself

1. These bacteria actually where the ones used to discover that DNA is the genetic material in 1943.

2. When bacteria react with capsular antibodies and water there is a quellung reaction – the capsule swells.

3. The most typical test for S. pneumoniae is adding detergents. These activate autolytic enzymes and make the live cells lyse, so if you look at the plate there will be a clear zone.

4. The bacteria bind to an acute phase protein (C Reactive Protein, CRP) produced by the liver because of inflammation, also used to diagnose inflammation. CRP activates compliment so bacteria can be eaten.

5. Capsular Polysaccharide There are 83 different serotypes, and this is how the bacteria can be distinguished. FAVORITE USMLE QUESTION  what antigen allows you to serotype what bacteria? REMEMBER S. pyogenes  M protein, S. pneumoniae  capsule.

6. The huge number of serotypes is a problem, how do we vaccinate? Well there are only 22 that actually cause disease, so we vaccinate against these serotypes. #’s 10 and 22 cause 85% of cases. The vaccine’s been around for 45 years and it protects from pneumonia and complications.

What are S. pneumoniae’s virulence factors

Teichoic Acid, LTA, and peptidoglycan fragments are released when the cell lyses. They activate compliment and trigger cytokine release by the cells of the immune system. Pneumolysin is a cytolytic toxin. The capsule is antiphagocytic and prevents C3b from attaching and causing opsonization.

S. viridans

Although they mostly stick to teeth, they also adhere to the tongue and cheek. If they get into blood because lets say someone got a tooth pulled, and that person has damaged heart valves, you can see subacute endocarditis. This is very rare nowadays because dentists give antibiotics when they pull teeth and ask if you have heart disease in the family so they know they have to be careful. This is an example where the predisposing factors are known so the disease is eliminated.

Category: Microbiology Notes